Plasma Ignition and Combustion Assist System for Gas Turbine Engines

ABSTRACT

An ignition and combustion assist system and method comprising a plasma igniter and electronic driver unit for use with gas turbine engines operating under low air densities, reduced voltage conditions and overall pressure ratios of 3:1 to 7:1. The plasma igniter has an inner chamber housing a centrally positioned and electrically isolated electrode attached to an electrical lead, driver unit, and AC or DC power supply. The electrode features a corner positioned near an outlet end of the igniter, where a plasma arc ignites a fuel-air mixture creating a flame extending into a primary burn region of a combustor of the gas turbine. The driver unit is in two embodiments and configured with low-cost microsecond voltage wave time periods or energy-efficient nano-second pulses. The method uses the plasma igniter and the electronic driver units described herein separately with other components or together.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and benefit from U.S. provisionalapplication Ser. No. 63/153,022 filed 24 Feb. 2021, claimed under allapplicable sections of Title 35 of the United States Code including, butnot limited to, Sections 120, 121, and 365(c), and which in its entiretyis incorporated by reference into this application.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

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NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT

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INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC OR ASA TEXT FILE VIA THE EFS WEB SYSTEM

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STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINTINVENTOR

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BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to ignition systems for turbine engines. Moreparticularly, the invention is a plasma ignition system with anelectronic driver unit and one or more igniter components for use with avariety of gas turbine engine applications and particularly withminiature turbojets and miniature high-speed turbo generators withrelatively low pressure ratios from 3:1 to 7:1 overall.

Background Art

All combustion engines have an air-fuel mixture inside the combustorthat is ignited, the hot air generated in the combustor used to thenturn blades of a turbine, piston, etc. Current ignition systems such asspark igniters rely on multiple factors for combustion to take place:stoichiometry, gas pressure, timing of the spark generated, and thevoltage applied to create a sufficiently large spark must all becarefully calculated and calibrated to maximize performance andreliability.

One problem with current igniter systems is the fact that they aregenerally unsuitable for miniature lightweight engines, defined as thoseengines with diameters less than 16 inches and weights of less than 25lbm, such as those used with lightweight drones, miniature missiles,airborne jamming devices, etc. as the cost, size (exciter electronicslarger than a 3 inch cube) and weight (more than 1.0 lbm) of currentigniter systems render them unsuitable. Smaller applications such asdrones ideally need lighter and lower power components and accordinglyfavor ignition systems that are simple, lightweight, withrelight/restart capability and are flexible according to the specificapplication needs.

The need for improved combustion system operability (stability over awider range of fuel/air mixtures, at higher flame strain rates andshorter residence times), as well as limitations of prior art ignitersystems have created intense interest in research using plasma ignitersand suitable applications of this technology, including use withinternal combustion engine and dual fuel engines. For small gas turbinessuch as those used in drones, and which operate with under low pressureratios and low combustor inlet temperatures the performance of currentigniter systems is lacking. The cost and limited performance oftraditional spark ignition systems and pyrotechnic flare igniters haveresulted in the need for a better system for achieving light-off,including re-light, with low-cost components.

What is needed is a plasma ignition and combustion assist systemcomprising a plasma igniter and an electronic drive unit that can beused with small gas turbine engines and a method of using a plasmaigniter system having an ability to choose between more energy efficientapplications and those that are more cost effective. What is also neededis a plasma igniter that generates a continuous electrical arc. What isalso needed is an ignition system that can operate at lower voltagelevels than conventional spark systems and yet deliver a higher energyoutput to the combustor. What is also needed is a plasma igniter thatextends the plasma arc into the combustor's primary burn area, improvingfuel burning efficiency. What is also still needed is an ignition systemthat has multiple re-light capability.

DISCLOSURE OF THE INVENTION

A plasma ignition and combustion assist system and method of using aplasma igniter and an electronic driver unit with a gas turbine engineoperating under low temperature, low pressure ratios, and otherconditions inappropriate for conventional spark systems. The plasmaigniter and the electronic driver unit are lightweight and appropriatefor use in drones, and other applications that require multiple re-lightcapability, as well as low-cost and higher efficiency options. Theplasma igniter and the electronic driver unit in the method can be usedtogether or separately with other igniter and driver units.

In a first aspect of the plasma ignition system, the plasma igniter iscomprised of a substantially cylindrical igniter body having a lead endand an outlet end, with an inner wall defining a chamber between theends. An electrode having a proximal end and at least one of a conicaland cylindrical distal end is housed centrally inside the chamber so asto be electrically isolated from the inner wall, forming anapproximately annular air gap within the chamber around the electrode.The igniter body is electrically grounded to the combustor, or directlyto the igniter driver electronics via an insulated wire. A diameter ofthe electrode is between about 0.125 and 2.0 inches. The electrodedistal end is positioned towards the outlet end of the igniter body andis further formed with at least one corner having a corner radiusranging from zero to 0.15 inches. The corner in some embodiments isconfigured as a projection. An arc gap from the corner to the inner wallof the igniter body ranges from a shortest or smallest distance from thecorner to the inner wall to a shortest or smallest distance measured tothe inner wall at the outlet end. In some embodiments, the arc gap isbetween about 0.125 inches to about 0.75 inches, and in otherembodiments, the arc gap measures between about 0.04 and 0.5 inches. Anelectrical lead connects to the electrode to the driver unit and powersupply. An air feed through-hole in the igniter body allows air flowinto the air gap and exit the outlet end, forcing a plasma arc generatedat the arc gap into the primary burn region of a combustor of the gasturbine engine. In some embodiments, the air feed through-hole is sizedand shaped to support an air injection velocity ranging from about 50 to300 ft/sec.

In another aspect of the plasma igniter, at least one of a fuel feedport, which may be a simple orifice, and a fuel feed port and a fuelatomizing injector integral with the igniter body is included. In someembodiments, the fuel feed port is sized and shaped to control at leastone of a fuel velocity of a quantity of fuel entering the arc gapranging from about 5 to 300 ft/sec and an inlet pressure ranging from2.5 psia to 100 psia. In other embodiments, the quantity of fuelentering the annular arc gap enters as fuel droplets with a meandiameter greater than 80 microns.

In yet another aspect of the plasma igniter, the air feed through-holeis positioned between the insulator and the outlet end of the igniterbody, whereby air flow entering the air gap through the air feedthrough-hole forces an arc generated within the igniter body into theprimary burn region of the combustor.

In yet another aspect of the plasma igniter has an igniter body selectedfrom the group of igniter bodies including an extended length igniterbody and a truncated igniter body.

In another aspect of the plasma igniter system, the driver unitcomprises an input power controller, a voltage oscillator communicatingwith the input power controller, a transformer communicating with thevoltage oscillator and the input power controller, an on-off switchcommunicating with the input power controller, and a power sourceproviding at least one of alternating and direct current input to thedriver unit. The driver unit provides an output of voltage and currentto the electrode and is grounded to the engine or to the combustor. Theinput power is regulated, filtered and modulated by the input powercontroller. The voltage oscillator creates an electrical output waveformat a desired frequency and level. The transformer transforms theelectrical output waveform generated by the voltage oscillator andgenerates a voltage level and voltage rate of change sufficient tocreate an electric arc.

In yet another aspect of the driver unit, the voltage and currentsupplied to the electrode are transient and a voltage wave time periodis measured in at least one of nano-second pulses and micro-secondpulses in a repetitive cycle.

In yet another aspect of the driver unit, the oscillating voltage outputlevels at the electrode range between about 250 Vrms to 7000 Vrms.

In still yet another aspect of the driver unit, the direct current powersource with a voltage level between 10 Vdc and 120 Vdc to the driverunit provides current to a circuit generating a variable or constantfrequency voltage wave at about 10 kHz to 10000 kHz.

In yet another aspect of the driver unit, the input power controller isat least one of a passive circuit with a single state for input andoutput and a voltage and current regulation system.

In yet another aspect of the driver unit, a voltage level increase of100 to 1000 times the input voltage via a voltage transformer isproduced by either an inductive electrical coil or a set of energystorage capacitors to achieve the oscillating voltage increase.

In a method of using the plasma igniter system having a plasma igniterand an electronic driver unit, the method comprises the steps ofdetermining at least one of a desired size and weight of a plasmaigniter based on engine size, space availability or kinetic application,determining a desired igniter electrode operating life, determining adesired power efficiency of the plasma igniter system, maintaining apower source compatibility of the plasma igniter system with that of theengine, determining engine pressure ratios, and determining whether theplasma igniter and driver unit will be operational only at initialignition and start of the engine or at multiple times after initialignition and start of the engine.

In another aspect of the method, the step of determining engine pressureratios further comprises the steps of identifying engines having lowpressure ratios between 3:1 to 7:1, small volumetric flow rates below 15msec, and operating at temperatures below 400 Fahrenheit, and selectingelectronic driver units with voltage outputs appropriate for at leastone of the respective pressure ratios and volumetric flow rates, afterthe step of determining engine pressure ratios.

In yet another aspect of the method, the step of selecting electronicdriver units further comprises the steps of sizing the arc gap inaccordance to increased voltage requirements.

In still yet another aspect of the method, the method is used with aturbojet with thrust ranging from about 15 to 600 lbf.

In yet another aspect of the method, the method is used with aturbo-generator having a 5 to 100 kW electrical power output.

In yet another aspect of the method, the method is further comprised ofthe steps of operating the plasma igniter to sustain combustion orincrease combustion efficiency when conditions where mixing and reactiontimes are short or where the fuel-air mixture in the combustor burn zoneis outside conventional lean and rich flammability limits, after thestep of determining whether the plasma ignition system will beoperational at initial ignition and start of the engine only, ormultiple times after initial ignition and start.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the invention will become apparent from aconsideration of the subsequent detailed description presented inconnection with accompanying drawings, in which:

FIG. 1 is a side elevation cutaway view of a plasma igniter having atruncated body of a plasma ignition and combustion assist system.

FIG. 2 is a side elevation cutaway view of a plasma igniter having anextended body of the plasma ignition and combustion assist system.

FIG. 3 is a perspective cutaway view of the plasma igniter in FIG. 2,showing a direction of air flow into the air gap.

FIG. 4 is a perspective view of the plasma igniter in FIG. 2.

FIG. 5 is an exploded view of the plasma igniter in FIG. 4.

FIG. 6 is a perspective view of the plasma igniter in FIG. 1.

FIG. 7 is an exploded view of the plasma igniter in FIG. 6.

FIG. 8 is a front elevation view of the plasma igniter, showing oneembodiment having protrusions extending from a distal end of theelectrode towards an inner wall of the igniter body.

FIG. 9 is a perspective view of the plasma igniter in FIG. 1, shown in ause position with an outlet end of the igniter body directed at aprimary burn region of a combustor.

FIG. 10 is a side elevation cutaway view of the plasma igniter in FIG.2, shown in a representative embodiment with a plasma arc generated bythe plasma igniter and a flame caused by the plasma arc igniting fuel,the flame extending into the primary burn region of the combustor.

FIG. 11 is a representative electronic driver unit voltage and currentoutput to the plasma igniter.

FIG. 12 is a first representative electronic driver unit configurationfor use with a plasma igniter of the plasma ignition and combustionassist system.

FIG. 13 is a second representative electronic driver unit configurationfor a plasma igniter of the plasma ignition and combustion assistsystem.

DRAWINGS LIST OF REFERENCE NUMERALS

The following is a list of reference labels used in the drawings tolabel components of different embodiments of the invention, and thenames of the indicated components.

-   100 plasma igniter-   100 a truncated body igniter-   100 b extended body igniter-   10 igniter body-   10 a outlet end-   10 b lead end-   10 c outer wall-   12 retention cap-   10 e mount-   16 insulator-   18 lead wire retention material-   20 electrical lead-   20 a power end of electrical lead-   20 b electrode end of electrical lead-   22 crimp or braze joint or solder-   24 electrode-   24 a distal end of electrode-   24 b proximal end of electrode-   24 c terminal end or vertex of electrode-   24 d corner-   26 arc gap-   28 inner wall of chamber of igniter body-   30 air gap-   32 air feed hole-   34 fuel feed hole-   36 fuel feed line-   40 combustor-   42 primary burn region of combustor-   44 combustor recirculation zone-   50 electronic drive unit or electronic driver unit or driver unit or    drive unit or driver or unit-   52 on/off trigger-   54 input power (AC or DC)-   56 input power controller-   58 zero-voltage switching block-   60 flyback transformer-   62 high voltage output-   64 voltage oscillator and transformer block-   70 flame

DETAILED DESCRIPTION

A plasma ignition and combustion assist system for use with a gasturbine engine is comprised of a plasma igniter 100 in two embodiments,shown in FIGS. 1-10 and an electronic driver unit or drive unit ordriver unit 50, in two embodiments, shown in FIGS. 11-13.

The plasma igniter 100 is comprised of an igniter body 10 defined by anouter wall 10 c and a pair of opposed open ends. The igniter body 10 isgrounded to either the engine or a combustor 40 of the gas turbineengine. The igniter body 10 has an inner wall 28 defining anapproximately cylindrical inner chamber having a lead end 10 b enclosedby a retention cap 12 positioned over the lead end 10 b and an opposedoutlet end 10 a. The retention cap 12 is formed with a hole sized andshaped to receive an electrical lead 20.

An electrode 24 having an approximately cylindrical shape, with aproximal end 24 b and a distal end 24 a, is connected at the proximalend 24 b to an electrode end 20 b of the electrical lead 20 by a solderor braze or crimp joint 22, with the joined electrode-electrical leadpositioned inside the chamber through the hole in the retention cap 12.Note that the crimp joint 22 includes any other suitable connection anduse of the term “crimp joint” is not meant to be limiting. The electrode24 and electrical lead 20 are positioned centrally within the chamberand electrically isolated from the inner wall 28 of the igniter body 10by a lead wire retention material or retention material 18, typically aquantity of potting or solder sandwiched between the inner wall 28 andthe crimp joint 22, with a position of the electrical lead 20 secured bythe retention material 18 around a perimeter of the electrical lead 20and attached at one end to an interior side of the retention cap 12. Anapproximately annular air gap 30 is thus created between the inner wall28 and the electrode 24 positioned centrally within the chamber. One ormore through-holes or air feed holes 32 are formed into the igniter bodywall leading from outside the igniter body 10 and into the air gap 30. Asteady quantity of air generated by an air compressor is fed into theair gap 30 through the air feed hole 32. In some embodiments, analternative or additional fuel feed port 34 is also formed into theigniter body 10 and leading into the air gap 30. The air feed hole 32and the fuel feed port 34 are typically formed near the proximal end 24b of the electrode 24 however they may in fact be positioned anywherebetween the outlet end 10 a and the insulator 16 inside the chamber ofthe igniter body 10. The fuel feed port 34 may be configured as a simpleorifice or may include with the port 34 a fuel atomizing injectorintegral with the igniter body 10. If the fuel feed port 34 is present,a fuel feed line 36 supplying fuel into the air gap 30 is affixed to thefuel feed port 34. A power end 20 a of the electrical lead is attachedto a power supply input 54 for supplying power to the igniter 100.

FIG. 1 shows a truncated body embodiment 100 a of the igniter 100, withthe distal end 24 a extending beyond the outlet end 10 a and into aprimary burn region 42 of the combustor 40. The distal end 24 a isgenerally conical in shape, with a terminal end or vertex 24 cpositioned beyond the outlet end 10 a of the igniter body 10. A base ofthe distal end 24 a is further formed with a corner 24 d having a radiusbetween zero and 0.15 inches, with a smaller radius being preferable.The inclusion of the corner 24 d shortens a distance between theelectrode 24 and the inner wall 28, and thus creates a constriction ofthe air gap 30 at the outlet end 10 a. The corner 24 d may in fact be ofa uniform size and shape about the circumference of the distal end 24 aas in FIG. 1 or may be formed as a regular series of protrusions aboutthe circumference of the base of the distal end 24 a as shown in FIGS.6-8.

An arc gap 26, shown in the Figures as a squiggly line, is typically ashortest distance measured from the electrode 24 to the inner wall 28,as most clearly shown in FIG. 1 with the truncated body igniter 100 a,and may in fact be a same measurement as the air gap 30. Formation of aplasma arc can occur anywhere along a length of the electrode 24 to theinner wall 28, but the inclusion of the corner 24 d shortens the air gap30 distance and encourages formation of the plasma arc at the corner 24d to the inner wall 28 at that shortest distance. Hence, in thedrawings, the arc gap 26 shown as the squiggly line in the Figures isalso generally indicative of the plasma arc. This is advantageous forignition of the discharge as well as for ensuring the discharge isdisposed proximal to zones of at least one of a flame holding or acombustor recirculation zone 44. In all the Figures, the arc gap 26 isshown as emanating from the corner 24 d to the inner wall 28 at theoutlet end 10 a of the igniter body 10 and in FIG. 2, this distance isnot the shortest distance between the corner 24 d and the inner wall 28.FIG. 2 shows the location of the arc gap 26 when there is air flowpresent in the air gap 30, with the air flow extending the arc gap 26distance to the inner wall 28 at the outlet end 10 a, and thuseffectively moving the plasma arc formation towards the outlet end 10 a.If no air is actively being fed through the air gap 30, the arc gap 26in FIG. 2 would in fact be a shortest distance from the corner 24 d tothe inner wall 28 as shown in FIG. 1. Hence, the arc gap 26 distance maychange depending on whether there is air flow within the air gap 30 andexiting the outlet end 10 a. The inventors note that instead of havingthe corner 24 d be configured as a protrusion to encourage arcing atthat position, in other embodiments, the inner wall 28 could instead oralso be designed with a flange or other protrusion constricting theoutlet end 10 a of the igniter body 10 with a same end result ofencouraging plasma arc formation at the constriction point in theigniter body 10. The inclusion of the corner 24 d on the electrode 24 atthe outlet end 10 a and the air flow towards the outlet end 10 aoptimize the plasma arc formation at the outlet end 10 a such that aresulting flame 70 produced will be positioned in the primary burnregion 42 of the combustor 40. When the electrode 24 receives currentfrom the driver unit 50, the current jumps the space between theelectrode 24 to the inner wall 28 at the arc gap 26, and when there isair flow in the air gap 30, the flame 70 extends from the arc gap 26 andoutwards beyond the outlet end 10 a into the primary burn region 42.

The truncated body igniter 100 a may also have a fuel port 24 and fuelline 26 or be unfueled. For embodiments with the fuel port 24 and fuelline 26, a fuel-air mix from the fuel port 24 and the air feed hole 32enters and swirls through the air gap 30 around the electrode 24. Theplasma arc formed at the arc gap 26 ignites the fuel, creating the flame70, and the moving air pushes the arc and the flame 70 caused by theburning fuel beyond the outlet end 10 a of the igniter 100 a and intothe combustor 40. The igniter body 10 and the centrally positionedelectrode 24 are heated by the passage of electrical current throughboth components and this heating enhances the processes of evaporationand break-up of fuel injected into the air gap 30.

The outer wall 10 c of the truncated body igniter 100 a is furtherformed with a widened body mount 10 e, having a larger diameter comparedto the outer wall 10 c and sized and shaped to allow the igniter body 10to be more easily secured to the combustor 40.

In FIG. 2, an extended igniter body 100 b embodiment is shown, where thedistal end 24 a and vertex 24 c of the electrode 24 do not extend beyondthe outlet end 10 a of the igniter body 10. In FIG. 2, the corner 24 dis at the base of the conical distal end 24 a, but the corner 24 d isformed without any protrusions and the cylindrical electrode 24 at thecorner 24 d instead tapers to the vertex 24 c. The air gap 30 has auniform distance from the inner wall 28 to the body of the electrode 24.The arc gap 26 is shown from the corner 24 d to the inner wall 28 at theoutlet end 10 a of the igniter body 10 and is a longer distance ascompared to the arc gap 26 in the truncated body igniter 100 aembodiment in FIG. 1. As previously discussed, when the igniter 100 b isin use, air pumped into in the air gap 30 via the air feed hole 32 flowsout the outlet end 10 a of the igniter body 10 and extends the arc gap26 distance and thus the plasma arc to what is shown in FIG. 2, with thearc gap 26 and plasma arc indicated by the squiggly line.

In comparison to the truncated body igniter 100 a, typically theextended body igniter 100 b includes a fuel port 34 and fuel line 36along with the air feed hole 32.

The plasma igniter 100 and its embodiments 100 a 100 b are suppliedvoltage and current by the electronic driver unit 50, shown in FIGS. 12and 13 as schematics of two typical embodiments of the driver units 50and their component blocks. The driver unit 50 provides electrical powerto the plasma igniter 100 100 a 100 b. The driver unit 50 supports thegeneration of a sufficiently high electric field so that an electricalarc is generated between the electrode 24 and the inner wall 28 of theigniter torch body 10. The arc gap 26 and an electrode geometry aredesign parameters that affect the voltage level required. The larger thearc gap 26, the higher the voltage requirement. The timing fortriggering the plasma arc is not a critical requirement in these systemssince the frequency of the voltage wave and resulting arcs areeffectively continuous, as shown in FIG. 11. Wave time, measured as1/frequency, is shorter than the effective combustor residence time,typically 0.5 to 30 msec. Electrical power and voltage are dependent onthe required ignition energy for the combustor 40. Typical levels rangefrom 100 to 500 Watts (RMS power input to the system). The deliveredenergy at the arc gap 26 is typically 5 to 10 times lower than the powerinput. However, the inventors note these parameters may vary withoutlimitation and one with ordinary skill in the art would recognize thatchanges in these parameters outside of these typical levels do notnegate the novelty or usefulness of this invention.

FIG. 12 shows a first embodiment of the driver unit 50, with a schematicof the plasma ignition driver unit 50 with component blocks. The powerinput 52 can be either alternating or direct current. Most aircraftsystems use 28 Vdc power, and thus this is a preferred power input. Avoltage output 62 is dependent on the driver unit 50 output frequency,which can range from 5 kHz to 100 MHz. Voltage output levels will rangefrom 250 Vrms to 7000 Vrms. Peak voltage levels may be significantlyhigher. As shown in FIG. 12, there are two basic component blocks in thedriver unit 50, an input power controller 56 to regulate, filter andmodulate the input power 52 and a voltage oscillator 64 to create avoltage waveform at the correct frequency and level with a transformerto generate the high voltage required for arcing at the igniter 100. Thedriver unit 50 also has a triggering input 52, configured as an on/offsignal in a simplest form, and connections with safety fuses and EMIshielding to reduce electromagnetic noise from the driver unit 50.

FIG. 13 shows a second embodiment of the driver unit 50. In this system,a simple zero-voltage switching unit 58 is used along with a voltagestep-up transformer or flyback transformer 60. This circuitry is commonand used in applications where different DC voltage levels are needed.The transformer 60 steps up the voltage from the primary side voltage toa higher voltage necessary for generating plasma. This step up ratio canbe customized to best suit engine operating parameters and is controlledby the primary to secondary windings ratio of the transformer, the fluxcoupling and inductive effects. The circuit can be tuned to a particularfrequency, and a higher frequency is usually preferred for plasmaigniter systems. The simplicity and availability of low-cost componentsin the driver unit 50 make this a desirable means for plasma igniterpower input. The input power controller 56 could be a complex voltageand current regulation system, or a simple passive circuit with a singlestate for input and output.

The driver units 50 supply voltage and current to the plasma igniter 100100 a 100 b such that there is a transient rate of voltage risesufficient to create the electrical arc from the corner 24 d of theelectrode 24 to the inner wall 28 of the grounded igniter body 10. Thetwo types of driver units 50 used in this system include a low-cost ACdriver unit 50 with a microsecond voltage wave time period, and ahigh-cost, energy-efficient nano-second pulse driver unit 50. Eitherdriver unit 50 shown in FIGS. 12 and 13 can be configured to be low-cost(microsecond voltage wave) or energy-efficient (nano-second pulse),however the high-cost, energy-efficient driver unit 50 requiresrelatively more expensive electronics with faster switching capabilityand may or may not require a physically larger unit. Thus, eitherigniter 100 a 100 b may be driven by either electronic driver unit 50. Asingle engine may have multiple igniters 100 and driver units 50, with aminimum one igniter 100 and one driver unit 50 per engine. The type ofdriver unit 50 used is dependent on the desired cost and use of thesystem. The AC driver unit 50 is low-cost and more appropriate forengine ignition where activation is required for only a short period oftime, typically less than one minute. The nano-second pulse driver unit50 is more power-efficient. Either driver unit 50 can be used forignition, re-start and combustion sustainment, efficiency enhancement,and in conditions where mixing and reaction times are short or where thefuel-air mixture in the combustor burn zone is outside conventional leanand rich flammability limits. These systems are capable of multipleengine starts, unlike pyro-technic or flare devices used in conventionalmilitary unmanned systems and are desired for periods longer than 5minutes and where the system cost trades well against improved engineoperational envelope and fuel efficiency.

In short, the low-cost and high-cost (energy efficient) designs are bothDC powered. The circuitry to drive the arc formation is the differencebetween these driver units. In the low-cost design, the arc is producedby an AC voltage switching circuit with a simple step-up transformer. Inthe high-cost design, the voltage step up is done with high frequencyswitching components with different voltage amplifiers (solid statedevices). The high-cost design is used for higher efficiency and betterperforming arc characteristics, such as faster, easier arcing with moreactive ion generation.

For driver units 50 with a direct current power source with a voltagelevel between 10 Vdc and 120 Vdc, the driver unit 50 provides current toa circuit generating a variable or constant frequency voltage wave atabout 10 kHz to 10000 kHz. The input power controller 56 can beconfigured as a passive circuit with a single state for input and outputor as a voltage and current regulation system. For the driver unit 50 inFIG. 12, a voltage level increase of 100 to 1000 times the input voltagevia the voltage transformer 64 is produced by an inductive electricalcoil, or alternatively by a set of energy storage capacitors to achievethe oscillating voltage increase.

The plasma ignition and combustion assist system is applicable to a widerange of gas turbines. The full range includes both ground power systemsas well as aircraft engines. The system described herein is expected tobe lower cost than conventional spark ignition systems. In 2021, alow-cost system is approximately less than $500 USD and a high-costsystem is more than $2500 USD. In comparison, a conventional sparkignition system in 2021 costs between $4000-7000 USD. The plasma igniter100 100 a 100 b and driver unit 50 of the plasma assist system describedherein produces a continuous or pulsed arc that does not requireexpensive nor complex triggering electronics, and the voltage requiredto sustain the plasma arc is several factors lower than for sparkignition systems, which reduces the need for complex isolation leads andconnectors.

Plasma assist ignition is best applied to small or miniature gasturbines, which must be capable of operation with short combustorresidence times. These engines are characterized by low pressure ratiowith low combustor inlet pressure (pressure levels below about 125 psia)and temperatures below 400 F, and with overall residence time(volume/volumetric-flow-rate) below about 15 msec. Larger, higherpressure ratio engines having overall pressure ratios above 7:1 wouldbenefit from the plasma igniter 100 and driver unit 50 described hereinbut typically have higher voltage, single-spark, systems. Plasma assistsystems based on the plasma igniters 100 and the driver units 50described herein can benefit large ground power systems mostly byrunning continuously during operation thereby improving lean stabilityand allowing stable operation at conditions consistent with lowernitrogen oxides (NOx) and carbon monoxide (CO) emissions.

Typical engines for which the plasma ignition and combustion assistsystem is useful include the following:

1. Miniature turbojets with thrust ranging from 15 lbf to 600 lbf. Theseare generally used in flight systems (such as miniature missiles,surveillance aircraft/drones, airborne jamming devices and commercialdrones). These engines use a range of heavy distillate fuels includingJet-A, Diesel and JP-10. Current engine models suitable for use with theplasma assist system include the ATI070, and all derivatives of theB300STG turbojet and the ATI200, a 200 lbf thrust turbojet. For theseengines, the plasma ignition and combustion assist system would be usedfor ignition and engine start/operational envelope expansion with lowengine speeds and/or power levels at altitude. The plasma ignition andcombustion assist system allows for multiple starts and/or re-starts inflight; and

2. Miniature high speed turbo-generators in the 5 to 100 kW electricalpower output range. These generators are most commonly airborne powersystems used for small, unmanned commercial and military aircraft wherehigh power/weight is needed. Other suitable applications of the plasmaignition and combustion assist system include those where high power ina small, lightweight package is required, including small ground powerunits. Current engine models suitable for use with the plasma assistsystem include the ATI010e, a derivative of the B140TG and SP10e, whichare both 10 kWe turbo-generators, the ATI35e, a derivative of theB300STG 35 kWe turbo-generator, and the SP75e, a 75 kWe turbo-generator.For these engines, the plasma ignition and combustion assist system isused for ignition and starting. The system in this application allowsfor multiple starts and/or re-starts during flight.

Engines suitable for use with the plasma ignition and combustion assistsystem have relatively low overall pressure ratios of 3:1 to 7:1, andwhere plasma arcing in air is relatively easy due to low air densitiesand reduced voltage required for electrical arc initiation.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the scope ofthe present invention.

We claim:
 1. A plasma igniter for use with an electronic driver unit anda combustor of a gas turbine engine, the plasma igniter positionedinside the combustor and the combustor having a primary burn region, theplasma igniter comprising: an igniter body having a lead end and anopposed outlet end, with an inner wall defining a chamber between thelead end and the outlet end; wherein the outlet end of the igniter bodyis an open end of the chamber positioned near the primary burn region ofthe combustor; wherein the igniter body is electrically grounded toeither the combustor or directly to the igniter drive electronics by aninsulated wire; an electrode having a proximal end and at least one of aconical and cylindrical distal end housed centrally inside the chamberand electrically isolated from the inner wall of the igniter body;wherein the distal end is further comprised of a terminal end positionedtowards the outlet end of the igniter body; wherein the distal end isfurther formed with at least one corner having a corner radius rangingfrom zero to 0.15 inches; an arc gap of a predetermined distancemeasured from the corner to the inner wall of the igniter body; whereinthe arc gap is a distance selected from the group of distances includinga smallest distance from the corner of the electrode to the inner wallof the igniter body and a smallest distance from the corner to the innerwall at the outlet end of the igniter body; an approximately annular airgap between the electrode and the inner wall; an electrical lead havingan electrode end and a power end, the electrode end affixed inside theelectrode at the proximal end and the power end connected to the driverunit, whereby an electrical current is supplied to the electrical leadand electrode by the driver unit; and an air feed through-hole formedthrough the igniter body and into the air gap, whereby air flows intothe air feed through-hole and into the arc gap and out the outlet end ofthe igniter body.
 2. The plasma igniter in claim 1, further comprisingat least one of a fuel feed port and a fuel feed port with a fuelatomizing injector integral with the igniter body.
 3. The plasma igniterin claim 1, further comprising at least one protrusion on the distalend; wherein the corner is an outermost surface of the protrusion; and alargest circumference measurement of the distal end of the electrodeincludes the protrusion.
 4. The plasma igniter in claim 2, wherein theair feed through-hole is positioned between the insulator and the outletend of the igniter body, whereby air flow entering the air gap throughthe air feed through-hole forces an arc generated within the igniterbody into the primary burn region of the combustor.
 5. The plasmaigniter in claim 1, wherein the electrode is electrically isolated fromthe inner wall by an insulator and a quantity of lead retention materialselected from the group of retention material including potting compoundand solder positioned between the electrode and the inner wall.
 6. Theplasma igniter in claim 1, wherein the combustor is grounded to theengine and the engine is grounded to the electronic driver unit.
 7. Theplasma igniter in claim 1, wherein the arc gap is between about 0.125inches to about 0.75 inches.
 8. The plasma igniter in claim 1, whereinthe arc gap measures between about 0.04 and 0.5 inches; and wherein thediameter of the electrode is between about 0.125 and 2.0 inches.
 9. Theplasma igniter in claim 1, wherein the igniter body is an at leastsubstantially cylindrical metal body.
 10. The plasma igniter in claim 2,wherein the igniter body and electrode are heated by the electricalcurrent, whereby heating enhances evaporation and break-up of fuelinjected into the air gap.
 11. The plasma igniter in claim 2, whereinthe fuel feed port is sized and shaped to control at least one of a fuelvelocity of a quantity of fuel entering the arc gap ranging from about 5to 300 ft/sec and an inlet pressure ranging from 2.5 psia to 100 psia.12. The plasma igniter in claim 10, wherein the quantity of fuelentering the annular arc gap enters as fuel droplets with a meandiameter greater than 80 microns.
 13. The plasma igniter in claim 1,wherein the air feed through-holes are sized and shaped to support anair injection velocity ranging from about 50 to 300 ft/sec.
 14. Theplasma igniter in claim 1 wherein the igniter body is selected from thegroup of igniter bodies including an extended length igniter body and atruncated igniter body.
 15. A driver unit for use with an igniter havingan igniter body with an inner wall housing a centrally positionedelectrode in spaced apart relationship with the inner wall, the driverunit in electrical communication with the electrode, the driver unitcomprising: an input power controller; a voltage oscillatorcommunicating with the input power controller; a transformercommunicating with the voltage oscillator and the input powercontroller; an on-off switch communicating with the input powercontroller; and a power source providing at least one of alternating anddirect current input to the driver unit; wherein the driver unitprovides an output of voltage and current to the electrode; wherein thedriver unit is grounded to the engine or to the combustor; wherein theinput power is regulated, filtered and modulated by the input powercontroller; wherein the voltage oscillator creates an electrical outputwaveform at a desired frequency and level; and wherein the transformertransforms the electrical output waveform generated by the voltageoscillator and generates a voltage level and voltage rate of changesufficient to create an electric arc.
 16. The electronic driver unit inclaim 15, wherein the igniter is a plasma igniter comprising: anelectrode having a lead end and an opposed conical distal end with avertex at an apex of the conical distal end, and a corner formed at abase of the conical distal end, the corner having a corner radiusranging from zero to 0.15 inches; an igniter body having an inner walldefining a chamber with an outlet end and a lead end, the chambercentrally housing the electrode in spaced apart relationship with theinner wall such that the corner is positioned at the outlet end, and anair gap is formed between the electrode and the inner wall; an arc gaphaving at least one of a smallest distance from the corner to the innerwall and a smallest distance from the corner to the inner wall at theoutlet end; an electrical lead affixed to the lead end of the electrode,with the electronic driver unit supplying power to the electrical leadand to the electrode; and an air feed through-hole formed into theigniter body and into the air gap, whereby air flow entering the air gapthrough the air feed through-hole flows into the arc gap and out theoutlet end of the igniter body.
 17. The electronic driver unit in claim15, wherein the voltage and current supplied to the electrode aretransient; and wherein a voltage wave time period is measured in atleast one of nano-second pulses and micro-second pulses in a repetitivecycle.
 18. The electronic driver unit in claim 15, wherein theoscillating voltage output levels at the electrode range between about250 Vrms to 7000 Vrms.
 19. The electronic driver unit in claim 15,wherein a direct current power source with a voltage level between 10Vdc and 120 Vdc to the driver unit provides current to a circuitgenerating a variable or constant frequency voltage wave at about 10 kHzto 10000 kHz.
 20. The electronic driver unit in claim 15, wherein theinput power controller is at least one of a passive circuit with asingle state for input and output and a voltage and current regulationsystem.
 21. The electronic driver unit in claim 15, wherein a voltagelevel increase of 100 to 1000 times the input voltage via a voltagetransformer is produced by either an inductive electrical coil or a setof energy storage capacitors to achieve the oscillating voltageincrease.
 22. A method of using a plasma igniter with an electronicdriver unit, the method comprising the steps of: determining at leastone of a desired size and weight of a plasma igniter based on enginesize, space availability or kinetic application; determining a desiredigniter electrode operating life; determining a desired power efficiencyof the plasma igniter system; maintaining a power source compatibilityof the plasma igniter system with that of the engine; determining enginepressure ratios; and determining whether the plasma igniter and driverunit will be operational only at initial ignition and start of theengine or at multiple times after initial ignition and start of theengine.
 23. The method in claim 22, wherein the step of determiningengine pressure ratios further comprises the steps of: identifyingengines having low pressure ratios between 3:1 and 7:1, small volumetricflow rates below 15 msec, and operating at temperatures below 400Fahrenheit; and selecting electronic driver units with voltage outputsappropriate for at least one of the respective pressure ratios andvolumetric flow rates, after the step of determining engine pressureratios.
 24. The method in claim 23, wherein the step of selectingelectronic driver units further comprises the steps of: sizing the arcgap in accordance to increased voltage requirements.
 25. The method inclaim 23, for use with a turbojet with thrust ranging from about 15 to600 lbf.
 26. The method in claim 23, for use with a turbo-generatorhaving a 5 to 100 kW electrical power output.
 27. The method in claim22, further comprising the step of: operating the plasma igniter tosustain combustion or increase combustion efficiency when conditionswhere mixing and reaction times are short or where the fuel-air mixturein the combustor burn zone is outside conventional lean and richflammability limits; after the step of determining whether the plasmaignition system will be operational at initial ignition and start of theengine only, or multiple times after initial ignition and start.
 28. Themethod in claim 22, for use with a driver unit comprising: an inputpower controller; a voltage oscillator communicating with the inputpower controller; a transformer communicating with the voltageoscillator and the input power controller; an on-off switchcommunicating with the input power controller; and a power sourceproviding at least one of alternating and direct current input to thedriver unit; wherein the driver unit provides an output of voltage andcurrent to the electrode; wherein the driver unit is grounded to theengine or the combustor; wherein the input power is regulated, filteredand modulated by the input power controller; wherein the voltageoscillator creates an electrical output waveform at a desired frequencyand level; and wherein the transformer transforms the electrical outputwaveform generated by the voltage oscillator and generates a voltagelevel and voltage rate of change whereby an electric arc is generated inthe arc gap.
 29. The method in claim 22, for use with a plasma ignitercomprising: an igniter body having a lead end and an opposed outlet,with an inner wall defining a chamber between the lead end and theoutlet end; wherein the outlet end of the igniter body is an open end ofthe chamber positioned near a primary burn region of a combustor;wherein the igniter body is electrically grounded to the combustor; anelectrode having a proximal end and at least one of a conical andcylindrical distal end housed centrally inside the chamber andelectrically isolated from the inner wall of the igniter body; whereinthe distal end is further comprised of a terminal end positioned towardsthe outlet end of the igniter body; wherein the distal end is furtherformed with at least one corner having a corner radius ranging from zeroto 0.15 inches; an arc gap of a predetermined size measured from thecorner to the inner wall of the igniter body; wherein the arc gap is adistance selected from the group of distances including a smallestdistance from the corner of the electrode to the inner wall of theigniter body and a smallest distance from the corner to the inner wallat the outlet end of the igniter body; an approximately annular air gapbetween the electrode and the inner wall; an electrical lead having anelectrode end and a power end, the electrode end affixed inside theelectrode at the proximal end and the power end connected to the driverunit, whereby an electrical current is supplied to the electrical leadand electrode by the driver unit; and an air feed through-hole formedthrough the igniter body and into the air gap, whereby air flows intothe air feed through-hole and into the arc gap and out the outlet end ofthe igniter body.